Electrical apparatus such as individual switches and circuit breakers has long been available with locking devices. The door of an enclosure for a panel of switches, fuses, fuse holders and circuit breakers is often provided with a locking device. Providing such electrical equipment with a lock enables an electrician to interrupt power to a circuit for safety when working on that circuit. Also, it may be important to maintain power in a secure circuit, for example in the energizing circuit of a fire alarm or a food freezer. Use of a locking device guards such apparatus against being unintentionally operated to energize or deenergize the controlled circuit. The electrical apparatus is restored to what may be called a normal condition after the apparatus has been unlocked.
A simple padlock is often suitable for the locking purpose. As an example, both the enclosure of a panel of circuit breakers or fuses or fuse holders and the door of the enclosure may have projecting blades with aligned holes for a padlock, to lock the door shut. In similar equipment, a single blade projects from the enclosure of a panel, the blade passing through a slot in its door when closed. A padlock having a hasp extending through a hole in that blade blocks the door against being opened. Circuits controlled by such apparatus are protected, so that their energized or deenergized condition can not be changed while the padlock is in place.